1. Field of Invention
The present invention is directed to a method for enhancing the electrical resistivity of at least a region of a ceramic substrate. The present invention also is directed to a ceramic substrate having a region of enhanced electrical resistivity that is provided by the method of the invention. In addition, the present invention relates to thin film magnetic recording heads having inductive or magnetoresistive sensors and wherein the heads' electronic layers are disposed on a region of substrate of a ceramic material, the region having enhanced electrical resistivity relative to the remainder of the substrate and composed predominantly of the ceramic material. The region of enhanced electrical resistivity may be provided by the method of the present invention.
The method of the invention finds application in any field in which it is desirable to enhance the electrical resistivity of at least a region of a ceramic substrate. An example of a specific application of the present method is in the production of inductive and magnetoresistive (AMR, giant magnetoresistive, or spin valve) thin film magnetic recording heads.
2. Background of Invention
Ceramic materials are commonly used as a substrate in the production of inductive and magnetoresistive thin film magnetic recording heads. One subset of these ceramic materials is composed primarily of alumina (Al.sub.2 O.sub.3) and titanium carbide (TiC). A particular example of this type of ceramic material, commonly referred to as "AlTiC", includes about 60-80% by weight alumina and about 20-40% by weight titanium carbide, along with the possible intentional addition of other components in minor amounts. AlTiC provides excellent machinablity when subjected to the several shaping processes (slicing, lapping, polishing, etc.) used to form the recording head and its air bearing surface (ABS).
In general, thin film magnetic recording heads are produced as follows. The AlTiC or other ceramic material employed as the substrate in head production is typically provided in a wafer or "puck" form. A series of thin film layers are formed on a surface of the raw wafer, typically using lithography processes comprising one or more steps of seed layer deposition, photoresist, permalloy electroplating, resist stripping, seed layer removal, sputter coating, and removal of metallic and insulating films. The thin film layers formed on the wafer include the magnetic pole elements of the recording head, and the several thin film layers formed on the wafer are referred to collectively herein as the "electronic layer" to contrast that layer with the ceramic substrate material. The ceramic substrate material merely acts to support the electronic layer and does not participate electronically in the read/write process. After the electronic layer is formed on the wafer, the wafer is separated into single rows of devices, called rowbars, by executing spaced parallel cuts through the thickness of the finished wafer. Each rowbar will include a portion of the ceramic wafer and the portion of the electronic layer that has been formed thereon.
The configuration of the magnetic read and write poles within the electronic layer is critical to the proper performance of the head. After each rowbar is sawed from the finished wafer, it is mounted on a transfer tool and the freshly sawed edge of the rowbar is carefully lapped back to adjust the dimensions of the electronic layer. The lapped surface of the electronic layer of the rowbar, with the magnetic pole tips just exposed, becomes the operative end of the head that will fly closest to the rotating magnetic media, on the trailing edge of the magnetic recording head. After the lapping procedure, a number of air bearing surfaces are formed along an exposed ceramic surface of the rowbar. Each rowbar is then sawed into discrete units, each discrete unit including a portion of the ceramic wafer and the portion of the electronic layer formed thereon. Each discrete unit includes magnetic read and write poles and an ABS and is referred to as a magnetic recording head or a "slider". If a magnetic recording head is to be used in a disc drive, it is mounted to a suspension. The combination of the head and the suspension, known as a "head/gimbal assembly", is then incorporated into the hard disc drive. The suspension determines the pitch, roll, normal force, and height of the magnetic recording head relative to the magnetic media. Magnetic recording heads also may be utilized in video or tape devices, in which case they are not mounted to a suspension.
When a magnetic recording head is mounted to a suspension, it is oriented so that the ABS will face the magnetic media when the head/gimbal assembly is assembled into the disc drive. The ABS is designed to allow the magnetic recording head to aerodynamically fly over the magnetic media in microinch proximity as the media rotates, allowing the magnetic poles of the electronic layer to magnetically interact with the magnetic media. The suspension positions the magnetic recording head over the magnetic media so that the electronic layer is at a trailing edge of the magnetic recording head relative to the surface of the rotating magnetic media. The distance between a magnetic pole at the trailing edge of the magnetic recording head and the surface of the rotating magnetic media is referred to as the "flying height". In general, lessening the flying height increases the performance of the head.
The ceramic material from which the wafer is composed must have an electrical resistivity that is low enough to allow dissipation of static electricity accumulation during read/write performance. Wafers composed of ceramic materials having sufficiently low electrical resistivity, such as AlTiC wafers, are too conductive to allow the electronic layer to be built directly on the surface of the ceramic material. Therefore, in the production of magnetic recording heads using AlTiC as the wafer material, a thick (3-10 .mu.m) electrically insulating layer of alumina (typically amorphous alumina) is formed intermediate the ceramic wafer and the electronic layer. The electrically insulating layer is commonly referred to as an "undercoat" layer or "basecoat", and it must be deposited on a surface of the ceramic wafer before the electronic layer is formed. The process of undercoat formation is very costly. For example, the process of forming an alumina undercoat layer on an AlTiC wafer requires the use of a clean room and costly sputtering equipment, and the proper loading of the wafers into the sputtering apparatus is both time-consuming and critical to the process. During the sputtering process, the ceramic wafer is placed on a water cooled fixture. To effectively cool the wafer, indium-gallium liquid is manually applied between the wafer and water cooled fixture to establish intimate thermal contact. The indium-gallium liquid must be manually wiped off when the coating process is complete. The process of applying and removing the indium-gallium liquid is time-consuming, and any residue left on the wafer surface is a source of contamination for subsequent processes. After the undercoat layer has been deposited, the entire undercoat surface must be planarized, typically by lapping or chemical-mechanical polishing. The undercoat layer also must be adjusted to a specified thickness, surface roughness, and flatness before the undercoat layer's exposed surface receives the build up of the electronic layer thereon. The entire undercoat layer deposition process may take as long as 10 hours, depending on the thickness requirement.
FIG. 1 is a representation of a portion of a conventional magnetic recording head and shows the position of the head relative to the rotating magnetic media during read/write performance. The ABS 10 of the magnetic recording head 12 opposes the magnetic medium 14. The magnetic recording head 12 includes a AlTiC ceramic substrate 16, an alumina undercoat layer 18 disposed on the substrate 16, and an electronic layer 20 disposed on the undercoat layer 18. The arrow indicates the direction of movement of the magnetic medium 14 relative to the head. Thus, the electronic layer 20 is disposed on the trailing edge of the head 12. The general position at which the flying height of the magnetic recording head 12 above the magnetic medium 14 is measured is indicated as "A". The alumina undercoat layer 18, which typically is sputter deposited, is soft relative to the ceramic substrate material 16. For example, the measured hardness of an alumina undercoat layer is typically about half that of an AlTiC substrate. Therefore, during ABS and pole tip lapping, the alumina undercoat 18 and the overlying electronic layer 20, which includes the magnetic read and write poles, are worn away to a greater extent than the ceramic substrate 16. FIG. 2 depicts a magnetic recording head 12' having ABS 10', alumina undercoat layer 18', and electronic layer 20'. The magnetic recording head 12' is disposed above a rotating magnetic medium 14'. Preferential erosion of the undercoat layer 18' and the electronic layer 20' relative to the ABS 10' has occurred during ABS and pole tip lapping in the region generally indicated by "X". The preferential erosion of the magnetic poles relative to the substrate during ABS and pole tip lapping increases the vertical displacement between the surface of the ABS and the tips of the magnetic read and write poles, which is defined as the "pole tip recession". It follows that the flying height increases as the extent of pole tip recession increases. For example, the flying height A' of the magnetic recording head 12' of FIG. 2 is greater relative to that of head 12 of FIG. 1 by the extent of pole tip recession.
With flying heights now approaching near-contact levels, any increase in the pole tip recession may represent a significant fraction of the total distance between the recording head and the magnetic media. Thus, to ensure improved performance of the head, the extent of pole tip recession must be minimized. If the pole tips are too greatly recessed relative to the ABS, this may cause a degradation or a complete lack of signal. There is a desired minimum amount of pole tip recession that will cause the least loss of magnetic signal and also ensure that the magnetic poles will not contact the magnetic media. To determine whether pole tip recession is within the acceptable range, it is currently industry practice to inspect every magnetic recording head after ABS and pole tip lapping. This inspection process adds significant cost to the finished magnetic recording head, and a portion of the heads are discarded on failing inspection.
An additional problem inherent in using an alumina undercoat is that there is a difference between the thermal expansion coefficients of the alumina undercoat and of the AlTiC substrate. Moreover, the alumina undercoat retains a degree of internal residual stress upon application to the substrate. The combination of the undercoat's residual stress and the difference in thermal expansion results in warping of the undercoated wafer during the elevated temperature (typically 200-250.degree. C.) processes used to build up the electronic layer. As the magnetic recording head industry pushes for increased wafer diameter (currently moving from 4 inches to 6 inches, and even as great as 8 inches) and reduced wafer thickness (currently moving from 0.080 inches to 0.052 inches, and as thin as 0.030 inches), the degree to which alumina undercoated AlTiC wafers warp will become progressively worse. Rowbars sawed from warped wafers will be curled or bowed. If this bowing becomes excessive, it is difficult or impossible to perform pole tip lapping on the rowbar transfer tool. Magnetic recording heads cut from an excessively warped alumina undercoated wafer may have excessive and unacceptable geometric distortions. These distortions are commonly referred to as twist, camber, and crown.
Accordingly, a need exists for a method of producing magnetic recording heads in which the potential for pole tip recession is reduced and, consequently, the time and cost involved in producing the heads also is reduced. In addition, a need exists for a method of producing thin film magnetic recording heads that will not result in warping of undercoated wafers on build up of the electronic layer, and will not result in bowing of rowbars sawed from such undercoated wafers, thus reducing the extent of any twist, camber, or crown.